1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same. In particular, it relates to a semiconductor device in which short-distance wiring layers in functional blocks and long-distance wiring layers for connecting the functional blocks are formed to have optimum structures suitable for their objects, respectively, without increasing the number of manufacturing steps and the number of wiring layers.
2. Description of Related Art
In accordance with progress in miniaturization, operation speed and integration of semiconductor devices (integrated semiconductor circuits), delay of signal transmission due to wiring layers increases. The signal transmission delay due to the wiring layers is determined by a product value of capacity and resistance of the wiring layers.
The capacity of the wiring layers is determined by a distance between the wiring layers, a thickness of the wiring layers and a dielectric constant of an insulating film between the wiring layers. The distance between the wiring layers can be enlarged by forming a multilayer wiring, which reduces the capacity. However, the formation of the multilayer wiring increases the number of manufacturing steps, raises production costs and thus decreases the yield. If the wiring layers are thinned, the capacity between the wiring layers can be reduced but resistance of the wiring layers increases. Further, since current density increases, a material for the wiring layers must be low resistive and highly electromigration-resistant. The wiring capacity can be further reduced by using a layer of low dielectric constant. Moreover, the resistance of the wiring layers can be reduced by forming thick wiring layers with a low resistive wiring material.
Considering the circumstances mentioned above, low resistive and electromigration-resistant copper and an insulating film of low dielectric constant have been used in the field of high performance semiconductor devices as shown in FIG. 1. Further, in a short-distance wiring layer lying in an undermost part of the multiplayer wiring, ON resistance of transistors connected in series is greater than the wiring resistance. Therefore, the short-distance wiring layer is formed thin in view of importance of the wiring capacity. On the other hand, a low resistive transistor having high driving capability is used in a long-distance wiring layer. Therefore, thick wiring layers are formed at a greater distance therebetween in view of importance of the wiring resistance.
FIG. 1 shows a semiconductor substrate 1 on which semiconductor elements have been formed, an interlayer insulating film 2, thin short-distance wiring layers 3, an interlayer insulating film 4 of low dielectric constant, middle-distance wiring layers 5 having a moderate thickness and thick long-distance wiring layers 6 used for connecting the functional blocks.
Even if the wiring structure is optimized as described above, it is getting difficult to transmit a signal of several GHz or more through the entire distance of a chip, especially in the long-distance wiring layer, because of limitation of the wiring resistance and the wiring capacity.
In order to transmit a signal of high frequency of several GHz or more through a chip size distance of about 1 cm, it is desired to eliminate RC delay by using a transmission line such as a microstrip line or a coaxial cable (coaxial line) and realizing impedance matching at the end. This may be achieved by forming a multilayer wiring by a conventional method and then forming a transmission line. However, this method inevitably increases the number of manufacturing steps to form the multilayer wiring and the transmission line.